1. Field of the Invention
This invention relates to liquid crystal displays in which a pleochroic fluorescent dye is dissolved in a liquid crystal host material, the dye (fluorophor) being highly anisotropic in absorption and emission.
2. Description of Related Art
When such a dye is dissolved in a nematic host material, its "long" molecular axis is aligned with the director of the liquid crystal, and any orientation adopted by the liquid crystal can also align the dye molecules. Control of the dye emission and absorption can therefore be achieved by altering the alignment of the liquid crystal molecules by the application of an external electric field.
Fluorescence is a three-stage process initially involving the absorption of radiation at a specific wavelength, followed by relaxation to the lowest excited state, and ending with re-emission occurring at a longer wavelength. The whole process is very fast, typically 1-20 ns. In an anisotropic fluorophor, the emission and absorption dipoles are directionally related to the geometry of the dye molecule. Reorientation of the molecular axes with respect to the plane of polarization of the incident radiation results not only in a change in the absorption but also a corresponding change in the fluorescence.
Order parameters for absorption and fluorescence, defined in terms respectively of the optical densities and the fluorescence intensities resolved into parallel and perpendicular components with respect to the liquid crystal alignment, may be used to range fluorophors into an order of merit for display performance.
In a fluorescent display, the fluorescence may be stimulated by light absorbed directly by the fluorophor. For organic fluorophors this direct excitation results in a rather small difference in wavelength between the stimulating and the emitted radiation. Alternatively the fluorophor may be stimulated indirectly. In this case, the incident radiation, at a shorter wavelength, is absorbed by the liquid crystal host material and the stimulating energy is internally transferred to the fluorophor. There is then a large difference in wavelength between the stimulating and the emitted radiation. In a such display, where fluorescence results from energy transfer, the stimulating radiation may be low-energy ultraviolet (and hence invisible) which is absorbed strongly by the liquid crystal host material, while the fluorophor emits light in the visible part of the spectrum. This leads to a much more acceptable colour difference in the display.
Certain perylenedicarboxylic diesters are especially effective as pleochroic fluorescent dyes in this mode of indirect excitation. In particular, di(4-nonylphenyl) perylene-3,10-dicarboxylate and/or perylene-3,9-dicarboxylate has been described in GB No. 2169092A and in "Anisotropic fluorophors for liquid crystal displays" by R.L. Van Ewyk & al. Displays, October 1986, 155-160 as having excellent properties of fluorescence and order parameter. Both of these descriptions, published after the priority date of this application, are incorporated herein by way of reference.
It has now been found that displays containing the nonylphenyl diesters suffer from fading of the fluorescence after long exposure to ultraviolet and thus require uneconomically frequent replacement.